1. Field of the Invention
The present invention relates to a drilling rig for offshore use allowing for a lightweight, openwork structure, thereby eliminating the seaworthy requirement of the rig and allowing for versatile positioning of the drill mechanism. The present invention has been found to be particularly useful in the jack-up drilling rig art, and, hence, will be discussed with particular reference thereto. However, the present invention is applicable to other types of drilling rigs requiring lightweight, inexpensive structure as well as flexibility in positioning the equipment mounted on the structure.
2. Description of the Prior Art
A mobile jack-up drill rig is the most stable, versatile and economical offshore drilling unit for operating in water depths of fifty to four hundred feet. In all but the worst sea conditions, a jack-up rig is a stable platform from which drilling operations can be performed efficiently well above the top of the waves. In a moderate storm, a drill ship or a semisubmersible usually must shut down drilling operations due to the high roll angle and pitch angle caused by the wind and waves. A mobile jack-up drilling rig is stable because it is set on the sea floor, which is not affected by the surface sea conditions. It is versatile because it is not limited to any one bottom condition, water depth or geographic location. A mobile, offshore, jack-up drilling rig of the prior art has one disadvantage in that it becomes less competitive economically to build for water depths greater than four hundred feet. To increase the operating depth of a jack-up drill rig using present technology, the distance between the legs is usually made greater, thereby necessitating the addition of more steel between the legs. Additionally, as more steel is added to the hull and to lengthen the legs, more steel must be added to the legs to support the extra weight of the hull and the extra length of the legs.
Several types of jack-up drilling rigs have been known and used before, and typical examples thereof are shown in U.S. Pat. No. 3,183,676, issued May 18, 1965, to R. G. Le Tourneau; U.S. Pat. No. 3,466,878, issued Sept. 16, 1969, to N. Esquillan et al; and U.S. Pat. No. 3,093,972, issued June 18, 1963, to M. R. Ward, Jr. None of these devices, however, teach either a drilling, workover, or crane openwork jack-up rig that is nonseaworthy.
Several types of circular orienting systems have been known and used before, and typical examples thereof are cranes which rotate on an upper circular skid rail and well treatment facilities that mount on an ancillary portion of the hull. None of these teach the use of a curved skid rail in drilling operations to locate drilling equipment.